Nanotechnology Spotlight

Researchers demonstrate smallest possible carbon nanotube

(Nanowerk Spotlight) Since their discovery, the single-walled carbon nanotube (SWCNT) has evolved into one of the most intensively studied materials. A SWCNT can be regarded as a monolayer of a graphene sheet rolled up to form a seamless cylinder with axial symmetry and in general exhibiting a spiral conformation called chirality. Chirality is defined by a single vector called the chiral vector (n,m). This chiral vector is dependent on the orientation of the tube axis with respect to the hexagonal lattice. SWCNTs with different chiral vectors have dissimilar properties such as optical activity, mechanical strength and electrical conductivity.

An obvious question that has been around since their discovery, but so far has not been satisfactorily answered, is how small the smallest SWCNT is. Theoretical calculations predict that the smallest diameter for a stable SWCNT is around 0.4 nm, and there are three possible structures that correspond with this value - chiral vectors (5,0), (3,3) or (4,2). Many efforts have been made to produce the smallest SWCNT and identify its atomic structure, but the techniques used were not accurate enough. Consequently, until now, the diameter and structure of the smallest possible carbon nanotube have remained in doubt.

Not anymore, though. Researchers in Japan, affiliated with Sumio Iijima, the discoverer of carbon nanotubes, have successfully synthesized the smallest SWCNTs with a diameter of 0.4 nm by thermal decomposition of ferrocene molecules inside commercial-grade SWCNTs with a diameter of 1.1 nm. Apart from the scientific aspects of these findings, using the inner space of a carbon nanotube as a reaction cell appears to be an intriguing approach to fabricating new structures which would be unstable on their own.

"We were able to produce the smallest carbon nanotube with a diameter of ∼ 0.4 nm as an inner tube of a double-walled CNT and successfully identified their chiral indices" Dr. Lunhui Guan and Dr. Kazutomo Suenaga explain to Nanowerk. Using our technique, the carbon network of the smallest carbon nanotube can be faithfully imaged – as a moiré pattern – with hardly any error in measurement and therefore the chiral index assignment can be performed beyond a doubt."

Suenaga, who heads the Nanoscale Characterization Team at the AIST Carbon Center, and Guan, who is first author, together with Dr. Sumio Iijima report the first direct evidence for the smallest carbon nanotube and unambiguous assignment of the chiral index – which is (3, 3) – in a recent paper in Nano Letters ("Smallest Carbon Nanotube Assigned with Atomic Resolution Accuracy").

(a) HR-TEM image of the cap region of (3,3)@(10,6) DWNT, (b) simulated images, and (c) schematic model. (d) The inner cap structure can be modeled as half a C20 fullerene consists of six adjacent pentagons (shown in yellow). (Reprinted with permission from American Chemical Society)

"Needless to say, in order to assign the smallest SWCNT you first need to obtain a suitable sample that is not too difficult to identify" says Guan. "Free-standing SWCNTs with the smallest diameters are intrinsically unstable under electron beam irradiation and supposed to be quite sensitive to oxidation from the surrounding atmosphere. Also those nanotubes confined in thick multi-walled carbon nanotubes or lying on the supporting amorphous carbon film can always disturb observation. To bypass these problems, we directly synthesized the smallest SWCNTs by pyrolysizing ferrocene molecules (FeCp2) inside commercially available high-pressure CO conversion (HiPCO) SWCNTs, which have a mean diameter of ∼1.1 nm."

In previous works, several research groups claimed the synthesis of the smallest carbon nanotubes with diameters of ∼0.4 nm. These reports were controversial because none of the authors gave any direct evidence for the atomic structure of the smallest carbon nanotubes. They simply measured the diameter as a distance between two dark lines associated with tube walls in conventional high-resolution transmission electron microscopy (HR-TEM) images.

Guan and Suenaga cautions that HR-TEM image simulation has already revealed that the simple measurements of diameters with a ruler exhibit systematic and substantial deviation from the true diameters, which indeed undermines the reliability of these reported findings. Therefore the existence of the smallest nanotube has been awaiting its experimental proof.

"We have devoted significant efforts to developing a characterization technique by means of HR-TEM which enables us to identify the carbon nanotube structure with atomic resolution accuracy" says Suenaga. "As a result of our work, the carbon network of the smallest carbon nanotube can now be accurately imaged with hardly any error, which allows us to unambiguously assign the diameter and structure of the smallest SWCNTs."

With the help of a state-of-art microscope, a field emission TEM equipped with a post-specimen spherical aberration coefficient (Cs) corrector (CEOS), the AIST team was able to assign the chiral indices beyond a doubt. They also experimentally address and propose the possible cap structures of the smallest SWCNTs. The cap structures have never been experimentally determined so far.

Guan and Suenaga explain that inner tubes produced in his team's method are always capped. "There is no iron particle (catalyst) found near the cap. The caps of SWCNTs can be associated with the related half-fullerene structures, which are composed of pentagons and hexagons. Although a (5,5) (0.68 nm in diameter) SWCNT should have a cap structure of half a C60 fullerene (six pentagons and ten hexagons), a (3,3) SWCNT (0.41 nm in diameter) can be capped by a half dodecahedron (C20). Much attention has been paid to the cap structure in an effort to reveal the growth mechanism of SWCNT, since the structure of a cap may determine the chirality of the nanotube."

The difficulty in observing the cap structure, considered as the most labile part of the SWCNT, lies in its instability during the observation as well as the resolution problem. Guan and Suenaga point out that, especially for the possible smallest C20 fullerene, it has been found only in gas-phase with a lifetime of only 0.4 ms. "In contrast to a theoretical prediction, the (3,3) nanotube is rather unstable and extremely sensitive to the electron beam and, therefore, may not survive alone without the protection of outer nanotube" he says.

The successful imaging of the smallest (3,3) SWCNT and its possible cap structure of the half C20 could lead to a better understanding of the growth mechanism of the smallest SWCNT. In particular, three application areas could benefit from this work:

1) The AIST team provided a template method for growing carbon nanotubes where the diameter of the newly formed inner carbon nanotube was exclusively determined by the outer host carbon nanotube. The method could be suitable for the selective growth of SWCNTs in a controlled manner.

2) The successful observation of the unstable smallest SWCNT, even its cap, implies that SWCNTs can act as sheaths for some unstable molecules, which could not exist alone.

3) By realizing a simple chemical reaction inside SWCNTs, the scientists have shown that these structures are ideal reaction cells for HR-TEM observations. This technique could be the basis for a possible future application of directly visualizing chemical reactions of single molecules with atomic resolution inside the microscope in real time.

In a previous Spotlight on Suenaga's work we reported about his team's work in shaping and connecting carbon nanotubes like water pipes (Nanotechnology pipe dreams).